Oil and gas well drilling in the United States conventionally involves the rotation of a drill string pipe with a rotary drill bit at the bottom end. An engine or motor at the surface turns or rotates the entire drill string while turning the rotary drill bit at the bottom of the borehole. A drilling fluid generally called drilling mud is pumped downhole and out through ports in the drill bit to bring rock cuttings to the surface. As the drilling mud leaves the ports in the rock bit, the mud picks up and carries the rock material loosened by the rock bit to the surface, and cools the bearings of the rock bit. The mud lubricates the sides of the borehole reducing the friction between the sides of the borehole and the rotating drill string. At times, circumstances make it desirable to drive the drill bit at speeds that differ from the speed of the drill string, or it may be that the drill string need not be rotated. A downhole motor may accomplish this by placing the downhole motor at or near the bottom of the drill string. The motor may be electric or hydraulic. If it is a hydraulic motor, it may be a turbine, a positive displacement vane, or a Moineau motor. All motors have a power section with a stator and a rotor which produce torque and rotation between them; thrust and radial bearings between stationary and rotating members; a flow path for the drilling fluid from the drill string to the drill bit which path may be through the power section, and through or partially through the bearings.
The bearings can be isolated from the drilling fluid, with its contaminants which are hostile to the function and life of the bearings, by seals; however, effective seals complicate the design and are notorious for creating torque losses and expensive repairs usually result when failure occurs. It must be appreciated that a method of extending the operating life of the motor is important because premature failure of the motor while downhole causes expensive downtime on the drilling rig.